The second is to allow html5 tags and attributes in tinyMCE otherwise the tags would be removed when switching between html view, yes you can just use the code view permanently which seems to be a favorite “solution” offered by many developers whenever there’s an issue with the WYSIWYG but I for one would much rather use the WYSIWYG any day to coding in a crappy textarea. As one guy mentions in Nicholas’ comments you can add tinyMCE settings via the tiny_mce_before_init WordPress filter. So I did this:

function extend_valid_html5($init) {
	// Standard attributes
	$atts = 'role|accesskey|class|contenteditable|contextmenu|dir|draggable|hidden|id|item|itemprop|lang|spellcheck|style|subject|tabindex|title';
	$html5Elements = array(
		'article[#|cite|pubdate]',
		'aside[#]',
		'audio[#]',
		'canvas[#]',
		'command[#]',
		'datalist[#]',
		'details[#]',
		'figure[#]',
		'figcaption[#]',
		'footer[#]',
		'header[#]',
		'hgroup[#]',
		'mark[#]',
		'meter[#]',
		'nav[#]',
		'output[#]',
		'progress[#]',
		'section[#]',
		'summary[#]',
		'time[#|datetime]',
		'video[#]'
	);
	if(!isset($init['extended_valid_elements'])) {
		$init['extended_valid_elements'] = '';
	}
	$init['extended_valid_elements'] .= str_replace('#',$atts,implode(',',$html5Elements));
	return $init;
}

add_filter('tiny_mce_before_init', 'extend_valid_html5');

NB: Needs more attributes I’m just going to add them when I need them for now.

Nicholas adds his filter to the themes function.php file but I placed both filters into a html5support.php plugin which just sits there activated making the filters available, that way I can have multiple themes and not bother writing it into every themes function.php file.